Theo đề bài ta có:

\(\frac{12+x}{17+x}=\frac{4}{5}\Leftrightarrow\frac{\left(12+x\right).5}{\left(17+x\right).5}=\frac{4.\left(17+x\right)}{5.\left(17+x\right)}\Leftrightarrow\left(12+x\right).5=4.\left(17+x\right)\)

\(\Leftrightarrow60+5x=68+4x\)

\(\Leftrightarrow5x-4x=68-60\)

\(\Leftrightarrow x=8\)